1. Field of the Invention
The present invention relates to a method for forming an end portion of a cylindrical member such as a metal cylinder or shell, and an apparatus therefor, especially the method and apparatus for forming the end portion of the cylindrical metal member by spinning to form a reduced diameter portion having an eccentric or offset axis on the end portion of the cylindrical member.
2. Description of the Related Arts
In Japanese Patent Laid-open Publication No.3-226327, disclosed is a method for forming an end portion of a cylindrical member (hereinafter, simply referred to as a cylinder) made of metal to form a reduced diameter portion on the end portion. According to the Publication, a spinning process is performed by supporting the cylinder with a chuck and rotating it about its axis, and moving a roller for forming toward the axis to reduce the diameter of the cylinder, thereby to form the reduced diameter portion having a neck portion and a tapered portion. In general, the spinning process is employed to form a plate into a shell. Likewise, a flange and neck portion can be formed by spin flow forming into a cylindrical can body, as disclosed in U.S. Pat. No. 4,563,887. Furthermore, a computerized spinning machine has been proposed in Japanese Patent No. 2,534,530.
Recently, it has been requested to form the reduced diameter portion on the end portion of the cylinder to be offset or eccentric to a main body of the cylinder. When the metal cylinder is used for an outer shell of a muffler of an automotive vehicle, for example, it has been requested to form the reduced diameter portion having an eccentric axis on the end portion of the shell. Also, when the metal cylinder is used for a housing of a catalytic converter, it has been requested to form the reduced diameter portion having the eccentric axis on the end portion of the housing, so as to locate the converter near an engine.
According to the prior methods for forming the cylinder or shell by the spinning process, the reduced diameter portion was formed to be coaxial with the main body of the cylinder, but the reduced diameter portion having the eccentric axis could not be formed on the end portion of the cylinder. In order to produce the cylinder like the shell or housing as described above, therefore, the portions corresponding to the main body and the reduced diameter portion were formed by press working, and then these components were connected together by welding or the like. According to these methods, however, the produced cylinder can not be expected to be so strong, comparing with that of the integral construction. Furthermore, they need the connecting process, different from the forming process, so that it is difficult to produce the cylinder by those methods, and it is almost impossible to produce the cylinder by the computerized forming process as described in the prior publication. As a result, the manufacturing cost of the cylinder shall be increased, comparing with the cylinder of the coaxial type formed by the spinning process.